U.S. patent number 4,720,918 [Application Number 06/870,622] was granted by the patent office on 1988-01-26 for razor blades.
Invention is credited to Francis R. Curry, Edwin L. Glasson, by Jadwiga Kozlowska, legal representative, Romuald Kozlowski, deceased, Joan Pumfrey.
United States Patent |
4,720,918 |
Curry , et al. |
January 26, 1988 |
**Please see images for:
( Certificate of Correction ) ** |
Razor blades
Abstract
A razor blade edge tip has a cross-sectional shape defined, over
up to 40 .mu.m from the extreme edge by the equation w=ad.sup.n, in
which w is the tip chord thickness in .mu.m at a distance d from
the extreme edge; a is factor of proportionality not greater than
0.8, and n is an exponent having a value in the range 0.65 to 0.75.
This results in a tip shape which is relatively thick very close to
the edge but whose overall cross-section is narrow, compared with
known tip shapes.
Inventors: |
Curry; Francis R. (Twyford,
Berkshire, GB2), Pumfrey; Joan (Reading, Berkshire,
GB2), Glasson; Edwin L. (Bracknell, Berkshire,
GB2), Kozlowski, deceased; Romuald (late of Hanwell,
GB2), Kozlowska, legal representative; by Jadwiga
(Hanwell, London, GB2) |
Family
ID: |
26284445 |
Appl.
No.: |
06/870,622 |
Filed: |
June 4, 1986 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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634110 |
Jul 18, 1984 |
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Foreign Application Priority Data
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Nov 19, 1982 [GB] |
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8233014 |
Nov 14, 1983 [WO] |
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PCT/GB83/00288 |
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Current U.S.
Class: |
30/346.55;
30/346.5 |
Current CPC
Class: |
B26B
21/58 (20130101); B26B 21/56 (20130101) |
Current International
Class: |
B26B
21/00 (20060101); B26B 21/56 (20060101); B26B
21/58 (20060101); B26B 021/54 () |
Field of
Search: |
;30/346.53,346.54,346.58,346.5,346.55 ;76/11R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Watts; Douglas D.
Parent Case Text
This is a continuation of application Ser. No. 634,110, filed July
18, 1984, abandoned.
Claims
We claim:
1. A razor blade having a cutting edge tip of a material which has
a higher yield strength than stainless steel, the cross-sectional
shape of which up to a distance of forty micrometers from the
extreme edge is defined by the equation:
in which w is the thickness in micrometers of the tip at a distance
d in micrometers from the extreme edge of the blade; a is a factor
of proportionality not greater than 0.8; n is an exponent having a
value in the range 0.65 to 0.75; and the width w obtained from the
said equation is reduced in inverse proportion to the square root
of the ratio of the yield strength of said tip material to that of
stainless steel.
2. A razor blade having a cutting edge tip of stainless steel, said
cutting edge tip being coated with a material having a greater
yield strength than stainless steel, the cross-sectional shape of
said tip up to a distance of forty micrometers from the extreme
edge being defined by the equation:
in which w is the thickness in micrometers of the tip at a distance
d in micrometers from the extreme edge of the blade; a is a factor
of proportionality not greater than 0.8; and n is an exponent
having a value in the range 0.65 to 0.75; and by the equation;
##EQU2## in which m is the ratio of the yield strength of the
coating material to that of stainless steel; and by the
equation:
in which h is the thickness in micrometers of the coating.
3. A razor blade having a cutting edge tip of stainless steel, the
cross-sectional shape of which up to a distance of forty
micrometers from the extreme edge is defined by the equation:
in which w is the thickness of micrometers of the tip at a distance
d in micrometers from the extreme edge of the blade; a is a factor
of proportionality not greater than 0.8 and n is an exponent having
a value in the range 0.65 to 0.75; and wherein the blade tip is
formed with facets at a distance between forty and one hundred
micrometers from the extreme edge, which facets converge toward the
edge at an included angle in the range 9.degree. to
111/2.degree..
4. A razor blade having a cutting edge tip of gothic arch
configuration, at least the tip of said blade including a material
which have a higher yield strength than stainless steel,
the cross-sectional shape of said blade up to a distance of forty
micrometers from the extreme edge being defined by the
equation:
in which w is the thickness in micrometers of the tip at a distance
d in micrometers from the extreme edge of the blade; a is a factor
of proportionality not greater than 0.8; n is an exponent having a
value in the range of 0.65-0.75; and
the blade tip is formed with facets at a distance between forty and
one hundred micrometers from the extreme edge, which facets
converge toward the edge at an included angle in the range
9.degree. to 111/2.degree..
5. A razor blade having a cutting edge, the cross-sectional shape
of said cutting edge being such that the tip thicknesses w at
distances d from the tip of said cutting edge lie within the
following ranges:
said cutting edge tip being of gothic arch configuration,
the cross-sectional shape of said blade up to a distance of forty
micrometers from the extreme edge being defined by the
equation:
in which w is the thickness in micrometers of the tip at a distance
d in micrometers from the extreme edge of the blade, a is a factor
of proportionality having a value in the range of 0.71-0.92, and n
is an exponent having a value in the range of 0.65-0.75.
6. A razor blade according to claim 2 wherein the gothic arch
configuration has been changed by removing a small amount of steel
from the tip surfaces by abrasive stropping after the final facets
of the blade have been formed by honing.
Description
This invention relates to razor blades and is particularly
concerned with the shaping of the cutting edge.
The invention resides broadly in a razor blade having a cutting
edge the cross-sectional shape of which within the first 40 .mu.m
measured back from the extreme edge is defined by the formula
w=ad.sup.n wherein d is the distance from the tip in .mu.m; w=the
tip width (or thickness) in .mu.m at a given distance d; a is a
factor of proportionality of about 0.8 and n is an exponent having
a value less than 0.75, and wherein the included angle between the
tip facets in the region from 40 .mu.m to 100 .mu.m from the
extreme edge is within the range 7.degree.-14.degree. and
preferably 9.degree. to 111/2.degree..
In the case of a stainless steel blade n is in the range 0.65 to
0.75 and a is in the range 0.71-0.92.
It has been found that blades having these tip characteristics
provide improved shaving on comparative shave testing, but are
sufficiently strong to give a reasonable useful life.
In order to convey a proper understanding of the nature of the
present invention, it is convenient to describe and illustrate the
background prior art in some detail. In the accompanying
drawings:
FIG. 1 is a greatly magnified view of a blade tip of typical, or
average shape;
FIG. 2 is a tip shape diagram illustrating the principle of
"tip-width" measurement;
FIG. 3 is a highly diagrammatic representation of the cutting of a
facial hair;
FIGS. 4 to 7 are cross-sections of various respective blades
currently marketed by a variety of manufacturers;
FIG. 8 is a view, like FIGS. 4 to 7, of the tip shapes described in
British Patent Specification No. 1465697.
FIG. 9 is a diagrammatic illustration of a blade tip stropping
operation.
FIGS. 10 and 10A are representations of blade tip forms.
FIG. 11 is a graph of tip widths versus distance.
Cutting edges on razor blades are sharpened by grinding a
succession of pairs of facets (usually three) of different included
angles onto a strip of steel by means of suitably arranged abrasive
wheels. The cross-section through such an edge is illustrated in
FIG. 1 with typical values for dimensions and angles shown, and is
customarily described as a "3-facet edge". While the final pair of
facets is being ground, (this stage is usually called "honing"),
strip deflection in the sharpening machine together with the
mechanical interaction between the steel and the abrasive particles
of the wheel, produces final facets which are usually not planar
but slightly convex. The curvature is a function of the type of
steel and abrasive wheel used, as well as the sharpening machine
setting parameters. Because of this convexity of the final facets,
the blade tip cross section in this region is customarily referred
to as "Gothic arched". The curvature prohibits precise geometrical
definition of this part of the blade tip by means of a single
parameter so that it is usual to characterise the shape by defining
tip thicknesses widths at various distances back from the edge. An
alternative method is to ascribe a mathematical equation to fit the
form of each half of the facet cross-section. These methods are
illustrated in FIG. 2.
During use, a razor blade is held in the razor at an angle of
approximately 25.degree., and with the edge in contact with the
skin, it is moved over the face so that when the edge encounters a
beard hair, it enters and severs it by progressive penetration,
aided by a wedging action. It is believed that the cut portion of
the hair (which is on average about 100 .mu.m diameter) remains
pressed in contact with the blade facets remote from the facial
skin surface for a penetration up to only about half the hair
diameter. Beyond this, the hair can bend and contract away from the
blade to relieve the wedging forces. The resistance to penetration
through reaction between hair and blade facets therefore occurs
only over about the first 50 .mu.m of the blade tip back from the
edge and the geometry of the blade tip in this region is regarded
as being the most important from the cutting point of view. This is
illustrated in FIG. 3.
It is clear that a reduction in the included angle of the facets
would correspondingly reduce the resistance to continued
penetration of the blade tip into the hair. However, if the
included angle were reduced too much, the strength of the blade tip
would be inadequate to withstand the resultant bending forces on
the edge during the cutting process and the tip would deform
plastically (or fracture in a brittle fashion, depending on the
mechanical properties of the material from which it is made) and so
sustain permanent damage, which would impair its subsequent cutting
performance, i.e. the edge would become `blunt` or `dull`.
In order to design a suitable shape for the blade tip which is just
strong enough to prevent such bending induced damage, an estimation
has been made of the magnitude of the bending stresses imposed
during the severing of a hair. From these values and a knowledge of
the yield strength of the steel from which the blade is made,
minimum dimensions can be calculated for the tip section. The
stresses imposed during cutting were assumed to arise from the
visco-elastic flow of saturated hair material past the blade
tip.
Blades currently produced have tip geometries with some dimensions
which are below these minimum values and are known to become dulled
by edge bending during the normal shaving life (which is on
average, approximately 10 days for a blade made from conventional
razor blade stainless steel).
We have now found that by careful control of the tip geometry in
specific regions 0-40 .mu.m from the edge, the overall
cross-section can be reduced so that cutting performance and
shaving satisfaction are improved, while retaining adequate
strength to resist edge bending damage and so maintain acceptable
durability.
The tip shapes of various manufacturers blades currently on the
market are shown in FIGS. 4 to 7, and FIG. 8 illustrates blade tip
forms as described in British Pat. No. 1465967.
These known blade tip shapes are compared with the preferred blade
tip shape of the present invention in FIGS. 10 and 10A.
In one form of the present invention, the blade tip cross-section
is first narrowed by grinding the three facets to smaller included
angles than those typified in FIG. 1. This produces a blade tip
whose cross-section is generally narrower throughout and,
importantly, in the 0-40 .mu.m distance back from the edge, which
is of particular interest during hair cutting. Such an edge is too
weak to withstand stresses during shaving and must be further
modified. This is achieved by adding what amounts to a fourth
sharpening stage. It is carried out using rotating interlocking
discs or spirals of leather or synthetic leather, (usually called
"strops") with abrasive material added to their peripheries. The
sharpened blades pass between the strops, which polish the facets,
removing a small amount of steel from their surfaces, and so
changing the "Gothic arch" dimensions. This stage is called
"abrasive stropping". Because of the flexibility of the strop
leather, allowing it to conform somewhat to the sharpened blade
tip, abrasive stropping increases the curvature of the final facet,
close to the edge, while having less effect on the facet shape
further back.
It has been found that when blades are sharpened with suitably
reduced facet included angles, followed by an appropriate abrasive
stropping treatment, the tip shape is changed so that the tip
widths close to the edge become larger than those on conventionally
sharpened edges, while the tip widths further away from the edge
remain smaller than those on conventionally sharpened edges. This
results in the blade tip close to the edge being stronger than
normal, so that it can better resist the bending stresses imposed
on it during hair cutting, while the reduced section further back
from the edge, presents less resistance to penetration during hair
cutting, so facilitating the cutting process.
The ultimate tip radius of the edge should be conventional, with an
average value of less than 1000.degree. A and preferably less than
500.degree. A as stated, for example, in Patent Specification No.
1,378,550 (U.S. Pat. No. 3,761,374), that is, within the normal
range for conventionally sharpened edges.
Blades in accordance with the invention have been found to have
superior shaving performance when compared with conventional blades
on a standard shaving test.
One form of blade in accordance with the invention and the manner
in which it is formed are described in detail below, by way of
example, with reference to FIGS. 9, 10 and 11, in which:
FIG. 9 is a diagrammatic illustration of a blade tip stropping
operation;
FIGS. 10 and 10A are representations of blade tip forms in
accordance with the invention, compared with the known blade tip
forms seen in FIGS. 4 to 8. FIG. 10A is a detail from FIG. 10 on a
larger scale; and
FIG. 11 is a graph of tip widths `w` at different distances `d`
plotted on logarithmic scales.
Stainless steel razor blade strip, of nominal composition 13% Cr,
0.6% C, was hardened and tempered in accordance with conventional
practice, and sharpened by grinding and honing to produce edges of
three facet configuration, as illustrated in FIG. 1, but with
included angles smaller than those conventionally manufactured. The
blades were passed between rotating strops of artificial leather,
whose surfaces contained fine alumina abrasive, in the manner of
conventional abrasive stropping, where the angle set on the strops
(which is the included angle between the tangents to the strops at
their point of intersection, as shown in FIG. 9) was in the range
30.degree.-34.degree.. The facets were provided with a metallic
coating of an alloy of chromium and platinum (applied in accordance
with U.S. Pat. No. 3,829,969) with a superimposed coating of
fluorocarbon material, (such as described in British Pat. No.
906,005).
The processes of grinding, honing and stropping are well known in
the art, but it will be understood that less conventional methods
could be employed for sharpening the tip, e.g. deforming the strip
between appropriately shaped dies or rollers, or by electrolytic or
chemical dissolution shaping or by ion bombardment shaping.
The blade tip cross-sections were measured using optical
interferometry. A blade is placed under the objective lens of a
metallurgical microscope fitted with a Michelson type
interferometer and viewed at a magnification of about 1000.times..
The interferometer is adjusted to produce fringes which are
oriented at right angles to the edge of the blade. The blade is
tilted at an appropriate angle so that the fringes are displaced to
reveal the topography of the blade facets. The fringe spacing is
adjusted so that fringe displacements can be readily measured at
various distances back from the edge. Knowing the angle of tilt,
the tip shape is calculated from the sum of these fringe
displacements, measured at corresponding positions on each side of
the blade.
The results of these measurements are shown in FIG. 10, in which
the spread of profiles of the preferred blade tips over the first
40 .mu.m are shown by solid shaded bands, and the spread of
profiles of known blades is indicated by the cross-hatched
bands.
In this specific example, the tip widths w at distances d from the
extreme edge were as set out below:
______________________________________ d (.mu.m) w (.mu.m)
______________________________________ 0.25 .20-.30 0.5 .34-.50
0.75 .53-.72 1.0 .71-.92 2.0 1.17-1.37 4.0 1.86-2.16 8.0 3.05-3.52
20.0 6.12-6.85 30.0 8.43-9.52 40.0 10.73-12.11
______________________________________
The geometry of this profile was re-plotted on a graph using
logarithmic scales for tip thickness as a function of distance from
the edge and the resultant plot is shown in FIG. 11, from which it
is seen that a straight line can be fitted to the plotted
points.
From the slope and intercept of the straight line, the tip shape
can be defined by the equation w=ad.sup.n in which a is a factor of
proportionality of about 0.8 and n an exponent having a value of
not more than 0.75, and more specifically within the range
0.65-0.75.
The known blades measured were found to have best fit straight
lines with exponents (or gradients) within the range 0.76-1.0.
The smaller gradient is a primary characteristic of the present
invention and results in the fact that the blade tip of the present
invention, compared with those of the prior art, is relatively
thick and strongly arched close to the extreme edge, but relatively
thin over the remainder of the tip.
The included facet angles in the region 40-100 .mu.m from the tip
are in the range 9.degree. to 111/2.degree. but making due
allowance for manufacturing tolerance could be in the range
7.degree. to 12.degree. or even 7.degree. to 14.degree..
It must be appreciated that the tip shapes described above are for
stainless steel blades and could be made substantially thinner for
harder blade materials such as sapphire, titanium carbide or
diamond.
To produce an equivalent tip shape from a material harder than
stainless steel, we reduce the corresponding tip widths in inverse
proportion to the square root of the yield strength of the harder
material in comparison with stainless steel. In the case of
diamond, for example, the tip widths would be approximately 40% of
those calculated for stainless steel.
Furthermore, the tip region of a stainless steel blade may be
coated with a material harder than stainless steel and having a
higher yield strength. In such a case the chord widths given by the
basic equation are reduced by adopting the modified formula:
##EQU1## in which m is the ratio of the yield strength of the
coating material to that of stainless steel.
Furthermore, in order to ensure the integrity of the steel
substrate, the value for w must also satisfy the equation w.sup.3
.gtoreq.(w-2h)a.sup.2 d.sup.2n, where h is the thickness of the
coating.
It will be understood by those skilled in the art that the blade
tips may, in each case, be coated with materials such as p.t.f.e,
which further enhance the cutting action. The thicknesses of such
coatings are, of course ignored for the purposes of calculating the
tip chord widths.
* * * * *